Phagocytosis in macrophages altered by mutation and induction is being studied. Mutants in general phagocytosis were isolated by linking daunomyxin to polystyrene beads and feeding the beads to a macrophage-like cell line which was thus killed. If the cells were mutagenized, some survived. Survivors were changed specifically in general phagocytosis since they ingested polystyrene beads without toxin slower than the parent, but were the same as the parent in daunomycin sensitivity and receptor-mediated phagocytosis. Induction of Fc-mediated phagocytosis resulted when another cell line, which lacks this phagocytosis, was exposed to dimethyl sulfoxide; DMSO removal reversed induction. Changes in the membranes of mutants and induced cells are being studied. A sensitive method of detecting double label in autoradiographs, was devised, and shown to quantitate differences in one or a few out of 500-1000 spots. 3H develops a different color in color film that do higher energy isotopes. Double label gels or chromatographs are analyzed, after exposure to color film, using a new data collection and computer analysis method. We showed that differences in virulence among three different Salmonellae result from differential complement activation via the alternative pathway; after activation the bacteria were ingested at differential rates by macrophages, presumably via the C3b receptor. Since these strains were constructed genetically to differ only in membrane lipopolysaccharides (LPS), LPS was assumed to control the activation. Now, using erythrocytes coated with purified LPS we show-that LPS alone can reproduce the differential complement activation and C3b deposition. The activation rate depends solely on primary structure, and not on differences in LPS density on the bacteria nor degree of polymerization of the polysaccharide. Since differences in activation are caused by changes in only two hydroxyls per six sugar residue repeating unit, this system provides a unique probe for the effect of structure on complement activation.